Hi Bertie
The Roaster is longer & wider than the Ron Champion book spec Locost.
Not sure of the measurements but 4" longer & 2" wider is lurking in the back of my mind ! ( I am sure someone will come up with the exact measurements
)
The upshot is, the Locost body work is too narrow for the Roadster.
All I can say is be very wary of bodywork being advertised as Lowcost/Roadster as it is unlikely to be a direct fit to both !!